Synthesis, Tribological and Hydrolysis Stability Study of Novel Benzotriazole Borate Derivative
Tribological and Hydrolysis Stability Study of Novel Benzotriazole Borate
Derivative. PLoS ONE 9(1): e83501. doi:10.1371/journal.pone.0083501
Synthesis, Tribological and Hydrolysis Stability Study of Novel Benzotriazole Borate Derivative
Xiong Liping 0
He Zhongyi 0
Qian Liang 0
Mu Lin 0
Chen Aixi 0
Han Sheng 0
Qiu Jianwei 0
Fu Xisheng 0
Jie Zheng, University of Akron, United States of America
0 1 School of Basic Science, East China Jiaotong University , Nanchang, Jiangxi , China , 2 Shanghai Institute of Technology , Shanghai, Shanghai , China , 3 PetroChina Lanzhou Lubricating Oil R&D Institute , Lanzhou , China
Benzotriazole and borate derivatives have long been used as multifunctional additives to lubricants. A novel, environmentally friendly additive borate ester (NHB), which contains boron, ethanolamine, and benzotriazole groups in one molecule, was synthesized by a multi-step reaction, and its tribological properties in rapeseed oil (RSO) were investigated by a four-ball tribometer. The hydrolysis stability of the additive was investigated by half-time and open observation methods, and the mechanism of hydrolysis stability was discussed through Gaussian calculation. The novel compound NHB showed excellent performance under extreme pressure, against wearing, and in reducing friction, and its hydrolysis time is more than 1,220 times, which is better than that of triethyl borate. The mass ratio of NHB is bigger than that of the mixed liquid of triethyl borate and ethanolamine. The lone electron of amino N atoms forms a coordination effect with the B atom to compensate for the shortage of electrons in the B atom and to improve the hydrolysis stability of NHB. The surface morphology and the traces of different elements in the tribofilms formed with 1.0 wt.% NHB in were detected with scanning electron microscopy(SEM), energy dispersive X-ray spectroscopy (EDX)and X-ray photoelectron spectroscopy(XPS). The results shown that the additive caused a tribochemical reaction with the steel ball surface during the lubricating process. A mixed boundary lubrication film that contains organic nitrogen and inorganic salts, such as BN, B2O3, FeOx, Fe-O-B, and FeB, was also formed, and the formation of the lubricating film improved the tribological properties of the base oil.
Funding: The work reported here was sponsored by Chinese Natural Science Foundation (No: 21063007); Jiangxi Provincial Natural Science Foundation (No:
20122BAB203011); Jiangxi Education Department Foundation of China (Grant No: GJJ13358). The funders had no role in study design, data collection and analysis,
decision to publish, or preparation of the manuscript
Competing Interests: Authors QJ and FX are employees of PetroChina Lanzhou Lubricating Oil R & D Institute. There are no patents, products in development
or marketed products to declare. This does not alter the authors9 adherence to all the PLOS ONE policies on sharing data and materials.
Pure base stock cannot meet all of the stringent requirements of
modern industry. For this reason, some functional additives must
be added to suit the working condition. Traditional lubricant
additives face enormous challenges with the growing requirements
of environmental protection for lubricating oil . Some organic
derivatives that contain tribologically active elements (P, S, N, Cl,
Zn, Mo, and B) are admixed to oils as additives at relatively low
concentrations . The main tribochemical reactions will occur
during contact among surface materials, base stock, and
lubricating oil additives in the boundary. The presence of reactive metals,
which are exposed by the wear process, can both catalyze
lubricant breakdown and initiate chemical interactions with the
molecules of the lubricant additives .
Boron-based lubricant additives have recently received
significant attention because of their wear-reducing and frictional
properties as well as low pollution . Some studies showed that
borate ester possesses robust extreme pressure and antiwear
properties, especially in low viscosity oil .
Some simple boron compounds, such as borate salt or borate
ester, have also been used as corrosion inhibitor, antioxidant,
friction modifier, as well as extreme pressure and antiwear additive
in environment-friendly base stock. These organic borate ester
compounds are directly dissolved in lubricating oils, but inorganic
borate salts, such as OLOA 9750 (potassium borate), are dispersed
as insoluble nanoparticles . The antiwear mechanism of B
compounds in extreme pressure conditions is due to the formation
of thin layers of boric oxide (B2O3) on the metal surfaces . Boric
oxide is converted to boric acid (H3BO3) upon exposure to humid
air, which is a layered material with a specific structure, whereby
the atoms are covalently bonded to each other, and the layers are
weakly bonded. When the layers are stressed, they shear and slide
over one another easily, providing low friction . Some
Ncontaining heterocyclic compounds, such as benzotriazole and
triazine, have been used as lubricating oil additive. These
compounds possess viable tribological performances. Li Jiusheng
et al.  designed and synthesized a novel B derivative of
benzotriazole by combining N, B, and O atoms in one individual
compound. Their study showed that both benzotriazole and alkyl
borate groups may simultaneously react on steel surfaces, and
these reactions may be activated by shear and/or thermal effects at
surface asperities, which explains the superior tribological
properties of the additive.
However, the low hydrolysis stability of borate salts and borate
esters is the biggest limitation for industrial application. Stability is
easily attacked by water due to the lack electrons of the B atom.
The attack will cause the loss of the effective additive B
component, which decreases the tribological performances of the
lubricating oil  and limits its practical application. Some
studies showed that the addition of nitrogen-containing materials,
such as amine, will improve the hydrolysis stability of borate salts
and esters. N-containing heterocyclic compounds are reported to
possess excellent extreme pressure and antiwear properties in
lubricating oil. The number of N atoms is the main factor that
affects tribological performance. The N-containing heterocyclic
compound contains N atoms, which possesses a lone electron pair
in the p orbital that will form a complex with the empty 2p orbital
of the B atom to reduce the possibility of attack by some
nucleophiles, such as water.
The ethanolamine and benzotriazole ethanol groups, which
increase the number of N atoms in the borate ester molecule, are
introduced in the synthesis of a novel borate ester (NHB) in the
present work. The effects of both benzotriazole substitute and
ethanolamine group at the alkyl borate part of the molecule on the
tribological performance of rapeseed oil (RSO) are investigated in
a four-ball machine. Finally, both roughness and the elemental
composition of wear scars on the steel balls after the tribological
tests are analyzed using scanning electron microscopy/energy
dispersive X-ray spectroscopy (SEM/EDS) and X-ray
photoelectron spectroscopy (XPS). The hydrolysis stability was tested using
the open method and half-time method, and the Gaussian
calculation was used to investigate the hydrolysis mechanism.
Additive Synthesis and Analysis Methods
2.1 Additive synthesis
The analytical reagents boric acid and ethanolamine were
obtained from Shanghai Shenbo Chemical Industry Co., Ltd.
Benzotriazole ethanol was synthesized in the laboratory, whereas
trimethyl borate, triethyl borate, and tributyl borate were
industrial products. Figure 1 shows the synthesis route of borate
Benzotriazole ethanol (0.1 mol), boric acid (0.1 mol), and
ethanol amine (0.2 mol) were added to a four-mouth flask
containing toluene as solvent and water-carrying agent; the
mixture was then stirred and heated. The reaction ended when
the generated water in the water separator was equal to the
theoretical yield. Then, vacuum distillation was performed, and a
yellow viscous liquid benzotriazole borate derivative (NHB) was
obtained. The production of NHB is about 26.8031 g.The
structure of the product was analyzed by infrared (IR) and
elemental analyses. The elemental analysis results were as follows
(data in parentheses are calculated values): C%, 48.60(49.17); N%,
23.71 (23.89); H%,6.92(6.88); and B%,3.77(3.69).
The base oil, natural RSO, used in the present study was
produced by Jiujiang Co., Ltd. (Jiangxi Province). Its physical and
chemical properties are as follows: 0.80 mg KOH/g; viscosity
index, 214; n40uC, 38.60; and basic N concentration, 0.1975 mg/g.
2.2 Analysis methods
2.2.1 Specimens and testing apparatus. All the steel balls
(w 12.7 mm) used in the test were made of GCr15 bearing steel,
with hardness of HRc 59 to HRc 61. A microscope was used to
determine the wear scar diameter (WSD) of the three lower balls
with an accuracy 60.01 mm.
The Chinese standard test method GB 3142-82, similar with
ASTM D2783, was used to evaluate the maximum non-seizure
load at a rotation speed of 1,450 rpm for 10 s at room
temperature. Friction and wear tests were examined on a
fourball test machine made in Jinan Testing Machine Factory of
China at a rotation speed of 1,450 rpm at different loads for
2.2.2 Hydrolysis stability. Hydrolysis stability was investi
gated by an open method. Borate ester with 10 ml 1.0 wt.%
concentration in liquid paraffin was added to a 25 ml dry glass,
and changes in turbidity were observed. The turbidity of the liquid
indicates the generation of insoluble boric acid solid. The time
spent for turbidity change was used as the hydrolysis stability time.
Hydrolysis stability was investigated by another method,
namely, the half-life method. Approximately 50 ml of water and
5 ml of glycerin were mixed in a glass, added with 3 to 4 drops of
phenolphthalein solution and 0.1 mol/l NaOH solution and then
mixed again. This mixture was then added into a beaker with 0.4
1.2 mmol borate ester and twice the amount of NaOH. The
solution was stirred continuously until the color changed from red
to colorless. The time spent for the change to occur was considered
the hydrolysis stability time.
2.2.3 Computing method. All the calculations were com
pleted at the Nanchang University computing workstation. The
geometrical configuration of borate ester in the hydrolysis process
was optimized with a program of Gaussian 03 density functional
method, in B3LYP/6-31G* level. The charge quantity and atom
distance of borate ester molecules were calculated to obtain the
reasonable space structure .
2.2.4 Worn surface analysis. WSD was analyzed with a
PHI-5702 type XPS. The radiation source was Mg Ka with
pass energy of 29.35 ev, and the binding energy of C1s (284.6 ev)
was used as a standard value. The SIGMA type field emission
SEM (Zeiss Company) was used to study the rubbed surface
Results and Discussion
3.1 PB value
The PB values of the RSO containing different concentrations of
NHB are shown in Table 1. The table shows that the addition of
NHB in RSO improved the extreme pressure properties of the
RSO to a large degree. It can improve PB value over 190% than
that of base oil when NHB concentration is 0.7 wt %, this
improvement indicated that NHB possesses good extreme pressure
property as additive in lubricating oil. The PB values increased
when the NHB concentration was less than 1.0 wt.%, but did not
increase at concentrations higher than 1.0 wt.%.
3.2 Antiwear performance
Figure 2 shows the WSD of RSO at an applied load of 196, 294,
392, and 490 N in different NHB concentrations. RSO showed
robust antiwear behaviors upon the addition of NHB in base oil at
different applied loads. The increase in the mass concentration of
NHB in the base oil and the decrease of WSD indicated improved
Figure 3 shows the WSD at different concentrations of NHB in
RSO at different applied loads. The WSD increased with the
increase of the applied load. The WSD of the samples containing
additive oil was far less than that of the blank RSO. Higher
3.3 Friction-reducing performance
Figure 4 shows the relationship between the friction coefficient
and the different additive concentrations at 196, 294, 392, and 490
N. The synthesized additive NHB could reduce the friction
coefficient at the range of the applied load. The friction-reducing
effect became smoother when the additive mass concentration was
more than 0.3 wt.%, and the friction coefficient of the base oil was
reduced close to 50% at 0.3 wt.% concentration at 392 N.
Figure 5 shows the relationship between the friction coefficient
and the applied load at different additive concentrations. The
different concentrations of the synthesized additive RSO all
reduced the friction coefficient in the applied load test. The
friction coefficient values were lower than those of the base oil at
the applied load from 196 N to 490 N, and the lubricating oil at
1.0 wt.% concentration possessed better friction-reducing property
at any applied load.
3.4 Hydrolysis stability
The turbidity of petroleum wax oil did not change after two
months at room temperature when 1.0 wt.% NHB was added.
The turbidity time of trimethyl borate and tributhyl borate was
5 min and 10 min, respectively. These findings indicated that
NHB possessed excellent hydrolysis stability due to the presence of
benzotriazole and ethanolamine, which contain the N element.
The lone pair electrons of the N atom made up for the
electrondeficiency of the B atom.
Borate ester was added with 0.2 g water to observe the turbidity
time of liquid in a constant temperature of 70uC and to shorten the
turbidity time. The change in time was marked as the hydrolysis
time. The components and results are shown in Table 2. The
hydrolysis time of NHB was 76,980 s, which was 1,220 times that
of triethyl borate, and also bigger than that of the mixture of
different concentrations of ethanolamine and triethyl borate.
Thus, to make up for the B electron deficiency of ethanolamine,
the inner coordination with borate was better than that of the
outer coordination with triethyl borate.
The root cause of borate ester hydrolysis results is the sp2
hybridization of the B atom . An empty 2p orbital exists and is
easily attacked by the nucleophile, which has a lone electron pair.
The attack can increase the bonding action between the empty 2p
orbital and lone electron pair. On the other hand, the water
molecule contains a lone electron pair that can attack the B atom
in the borate ester and then contribute to its hydrolization.
According to extant research, the hydrolysis process of borate ester
is accomplished in three steps: first, the borate ester is attacked by
water, then, the unstable tetrahedral complex is generated, and
finally, alcohol is desquamated.
3.5 Worn surface analysis
To understand the boundary lubricating mechanism of the
additive in the lubricating process better, this work used SEM to
study the worn surface of the steel ball. Figure 6 shows the SEM
results of the worn surface of the steel balls lubricated by RSO and
RSO with 1.0 wt.% NHB under the applied load of 392 N.
The SEM micrographs in the figure indicate that severe scuffing
occurred with the lubrication of base oil alone based on the grain
abrasion characteristics . The lubrication of RSO containing
1.0 wt.% synthesized additive NHB showed only slight frictional
tracks with no obvious deep furrows. This observation indicated
that NHB can improve the tribological properties of base oil. The
N and B active elements in the additive NHB  effectively
formed a layer of B nitride films to cover the surface of the friction
pair during the lubricating process, which thereby reduced
Figure 7 shows the steel ball surface elements content and
distribution analysis of EDS, which offers a more intuitive analysis
of tribochemical reactions on the steel ball surface during the
lubricating process. The elements on the steel ball surface were Fe,
Cr, and C when lubricated with RSO only as the material. The
worn surface of the steel ball showed B and N elements, which
only came from additive NHB when using 1.0 wt.% NHB as
additives in RSO. Fe and Cr element concentrations were lower
than that of the base oil during the lubricating process.
Table 3 shows the different elements on the worn surface. When
the worn surface had N, B elements, the antiwear properties
increased. This increase was because when NHB was introduced
to the base oil, the N, B active elements of NHB effectively
combined to form boron nitride film [9,15], covering the steel ball
worn surface to stop further wear.
3.6 XPS analysis results
The worn surfaces of the steel/steel pairs lubricated with the
RSO containing 1.0 wt.% NHB under 392 N were analyzed by
XPS. The analysis obtained more information on the
tribochemical reactions involved during the sliding process. Figure 8 shows
the results of the analysis.
The binding energies of C1s were 285.7 and 283.9 eV, which
corresponded to the CH, CO, COO in the additive and base
oil. Thus, the base oil and additive were adsorbed on the metal
surface. The binding energies of Fe2p were 710.9, 709.5, and
723.4 eV, which corresponded to iron oxides , which were also
supported by the binding energies of O1s at around 533 eV. The
binding energies of O1s were 529.5, 531.6, and 532.8 eV, which
corresponded to Fe2O3, FeOB, and FeO. The peak in 531.3 eV
of O1s corresponded to the COO of the base oil. The broad peak
of B1s from 191.0 to 192.4 eV was attributed to BOx and OFeB,
among others . These data indicated that the formation of the
B element was adsorbed on the surface with boron oxygen, which
suggests the presence of degraded borate ester on the top surface.
The binding energy of B1s at 189.7 eV corresponded to the BN
form, which suggests that the presence of a small amount of
degraded borate esters that reacted with steel produced the BN
compound. As for the N element on the surface, the binding
energy of N1s was located at 399.5 eV, which can be assigned to
the adsorption of the organic N-containing compounds on the
overlayer . The N1s spectrum showed two peaks at 399.9 and
398.7 eV, indicated the existence of inorganic N-containing
compounds. The binding energy of N1s were 399.8 and
397.9 eV, which corresponded to organic N  and BN,
respectively. Moreover, these compounds were also supported by
the binding energies of B1s at around 189.7 eV. Hence, the N
element of amine was adsorbed on the steel ball surface with
organic amine and BN, which were produced during the
tribological process whereby the NHB had a tribochemical
reaction with the steel ball surface.
Figure 8 XPS spectra of the characteristic elements of NHB on
the worn surface
In conclusion, the tribofilm containing N, B, O, and Fe
elements were formed on the worn surfaces under the lubrication
of the RSO that contained the NHB during the sliding process, as
confirmed through XPS spectra analysis. The resulting surface
protective films contributed to the significant reduction in friction
According to the above results, the tribochemical mechanism of
NHB as additives in RSO is discussed below.
First, the additive was adsorbed on the metal surface, taking on
competitive adsorption with RSO in the lubricating process .
The tribological reaction between metals produced a partially high
temperature that resulted in the tribochemical reaction of the
additive with the steel ball surface. The additive molecule
decomposed, and the ethanolamine group reacted with the metal
surface to form organic N and N-containing metal complex film.
Subsequently, the benzotriazole ethanol group was adsorbed on
the surface. The borate group reacted with the metal surface to
form inorganic boron oxygen film, which possessed a high degree
of hardness, contributing to the higher carrying capacity of the
tribological surface film. The inorganic and organic protective film
in the metal surface had a certain tribological performance.
Therefore, a mixed reaction boundary film existed in the worn
steel surfaces caused by the chemical composition, base oil, and
additive in the lubrication process. The lubricant film formation
improved the tribological properties of the base oil.
In short, the tribological mechanism of the synthesized borate
ester derivative showed that the additive underwent a chemical
reaction with the steel ball surface and then formed a complex
boundary lubricating film that contained organic N-containing
compounds and inorganic salts, such as FeB, Box, and BN, among
others. The formation of lubricating films improved the
tribological properties of the base oil.
Hydrolysis Stability Mechanism
The space structure of benzotriazole ethanol borate ester was
evaluated by Gaussian software Gauss03 Revision-E.01. The
space structure and atom label of NHB are shown in Figure 9.
Structure A had no optimization, whereas structure B was
optimized. The figure shows that N atoms in structure A undergo
planar movement, and in structure B, the N atom (labeled 21)
moves toward B atom (labeled 13). Thus, the N and B atoms may
form a coordinated bond that allows electrons to move from the N
atom to the B atom.
4.1 Analysis of atom charges
Table 4 shows the atom charges. When structure A was
transformed into structure B, the charge of N atoms (labeled 17
and 21) decreased, whereas the charge of N atoms (labeled 7, 8,
and 9) did not change significantly with their move from structure
A to structure B. Hence, the charge of N atoms in ethanolamine
flowed toward the B atom, but the charge of the N atoms in
benzotriazole did not flow toward the B atom.
4.2 Separation distance between the B atom and different N atoms
Table 5 shows the separation distance between the B atom and
different N atoms. The Table 5 shows that with the optimization
of the molecule, the N atom in amino moved closer toward the
center B atom, but the N atoms in benzotriazole were far away
from the center B atom.
From the above results, the Gaussian calculation showed that
the hydrolysis stability of NHB may be attributed to the lone
electron pair of amino N atoms that form a coordination effect
with the B atom to compensate for the shortage of electrons of the
B atom, as seen from the charge of the B and N atoms as well as
the distance between the N atoms and B atom.
The following conclusions can be drawn from above results:
(1) The antiwear and extreme pressure properties of NHB
increased with the increase of the additive concentration. PB
values increased with the increase of the NHB concentration
under 1.0 wt.%, but was not increase at concentrations higher
than 1.0 wt.%. The antifriction effect becomes smoother
when the additive mass concentration is more than 0.3 wt.%,
and the reduction of the friction coefficient of the base oil is
close to 50% at 0.3 wt.% NHB at 392 N.
(2) The addition of 1.0 wt.% NHB in petroleum wax oil did not
change the turbidity, even after two months. The turbidity
time of trimethyl borate and tributhyl borate are 5 min and
10 min, respectively. The hydrolysis time of NHB is more
than 1,220 times than that of triethyl borate and is more than
that of the mixed liquid of triethyl borate and ethanolamine
with any mass ratio. The lone electron of amino N atoms was
formed a coordination effect with the B atom to compensate
for the shortage of electrons in the B atom and improved the
hydrolysis stability of NHB.
(3) The NHB has a tribochemical reaction during the sliding
process, and the N element of amine is adsorbed in the steel
ball surface with organic amine and BN, whereas other groups
form inorganic salts. XPS spectra and EDS analysis show that
the complex tribofilms containing B, N, O, and Fe elements
are formed on the worn surfaces under the lubrication of the
RSO containing NHB. The resulting surface protective films
contribute to reduce the friction and wear significantly.
Conceived and designed the experiments: XL. Performed the
experiments: HZ QL.Analyzed the data: QJ FX. Contributed
reagents/materials/analysis tools: CA HS. Wrote the manuscrip:
1. Faiz US , Sergei G , Oleg NA ( 2012 ) Novel alkylborate-dithiocarbamate lubricant additives:synthesis and tribophysical characterization . Tribol Lett 45 : 67 - 78 .
2. Shah FU , Glavatskih S , Antzutkin ON ( 2009 ) Synthesis, physicochemical and tribological characterization of S-di-n-octoxyboron-O,O-di-n-octyldithiophosphate . ACS Appl Mater Interfaces 1 : 2835 - 2842 .
3. Didziulis SV ( 1995 ) An XPS study of the chemical interactions of the extreme pressure lubricant additive lead naphthenate with titanium and titanium compound surfaces . Langmuir 11 : 917 - 930 .
4. Yao JB ( 2006 ) Recent development of antiwear, extreme pressure (EP) additives for lubricant . Lubr Oil China 21 ( 3 ): 29 - 37 .
5. Hu XL , Liang GZ ( 2002 ) Hydrolytic stability of borate ester and its application . Mater Rev China 16 ( 1 ): 58 - 60 .
6. Spikes H ( 2008 ) Low- and zero-sulphated ash, phosphorus and sulphur antiwear additives for engine oils . Lubr Sci 20 : 103 - 136
7. Philippon D , De Barros-Bouchet M-I , Lerasle O , Le Mogne Th , Martin J-M ( 2011 ) Experimental simulation of tribochemical reactions between borates esters and steel surface . Tribol Lett 41 : 73 - 82 .
8. Jean MM , Nobuo O , Ali E ( 2008 ) Boron-based solid nanolubricants and lubrication additives . Nanolubricants 203-224.
9. Li JS , Xu XH , Wang YG , Ren TH ( 2010 ) Tribological studies on a borate ester containing benzothiazol-2-yl and disulfide groups as multifunctional additive . Tribol Int 43 : 1048 - 1053 .
10. Wang YG , Li JS , He ZY , Ren TH ( 2008 ) Investigation on phenyl-borated hydroxyalkyl -dithiocarbamate as multi-functional lubricating additive with high hydrolytic stability and antioxidation . Proc Inst Mech Eng, Part J: J Eng Tribol 222 : 133 - 140 .
11. Xue RT ( 2011 ) Synthesis, crystal structure and properties of organic-inorganic hybrid perovskite molecular materials , Ocean University of China (Doctoral Dissertation)
12. Wang Q , Zheng G , Ai JJ , Wei XJ ( 2011 ) Study on the hydrolytic stability of borate ester . Appl Mech Mater 84-85 : 499 - 503 .
13. He ZY , Xiong LP , Zeng XQ , Ren TH ( 2005 ) Synthesis and tribology study of bi-alkoxy mono- thiophosphate triazine derivatives as additives in rapeseed oil . Chinese Sci Bul 50 ( 12 ): 1174 - 1179 .
14. Zhang H , Tu ZW , Fu SF ( 2008 ) Synthesis and tribological characteristic of nitrogen-containing borate as lubricant additive . Lubri Eng China 33 ( 8 ): 94 - 97 .
15. Jia ZF , Xia YQ , Pang XJ , Hao JY ( 2011 ) Tribological behaviors of different diamond-like carbon coatings on nitride mild steel lubricated with benzotriazolecontaining borate esters . Tribol Lett 41 : 247 - 256 .
16. Jia ZF , Xia YQ ( 2011 ) Hydrothermal synthesis, characterization, and tribological behavior of oleic acid-capped lanthanum borate with different morphologies . Tribol Lett 41 : 425 - 434 .
17. Zheng Z , Shen GQ , Wan Y , Cao LL , Xu XD , et al. ( 1998 ) Synthesis, hydrolytic stability and tribological properties of novel borate esters containing nitrogen as lubricant additives . Wear 222 ( 2 ): 135 - 144 .
18. Zhou XD , Wu DM , Shi HQ , Fu X , Hu ZS , et al. ( 2007 ) Study on the tribological properties of surfactant- modified MoS2 micrometer spheres as an additive in liquid paraffin . Tribol Int 40 ( 5 ): 863 - 868 .
19. Liang P , Ren TH , Li J , Ma HB , Wu H ( 2010 ) Tribological behaviour of two ashless and phosphorous-free anti-wear additives and their synergetic effect with zinc dialkyldithiophosphates in mineral oil . Proc Inst Mech Eng Part J: J Eng Tribol 224 : 65 - 72 .